Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: dude on January 07, 2019, 01:31:09 pm
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Going over this 71 AB763 Super Reverb for a friend who wants to gig with it. Not sure what to replace for "preventive measures", don't want the kid's amp to cut out on a big gig, his biggest yet, 500 people club event.
Amp plays ok, most of the coupling caps have been replaced, the power tubes grids and plates replaced, bias checks out ok at -52vdc, plates 470v, no 1 ohm R's. There're a lot of old carbon comp R's don't know what to replace, values are within 20% but most drifted high. All the plates are 100k 1 watt, look very brittle, I want to replace them but only have 100K 1/2 watts, OK to use 1/2w?
All the e-caps in the dog hse have been replaced, look and measure good, no hum. But the first filter has two 220uf's in series instead of the stock 70ufs, so it's 110uf for first filter, 5AR4, 110uf ok?
Guess this is it, reflow joints, anything else I should do?
al
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Was hoping to get an answer on the at least the first e-cap, two 220uf in series, 110uf, a sovtek 5AR4 says 60uf max. Someone replaced the originals a while ago with two 220's.
Hoping to close this amp up tonight, thanks
Schematic says nothing about anything more than 1/2 watt plate R's but these are "old" carbon comp 1 watts, looks like they've
been there since day one..?
Hopefully, I can get this young kid his amp back soon, it's a free-be, for a friend of a friends son.
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> don't want the kid's amp to cut out on a big gig
Then get 2 Watt 100K.
At happy idle, there's a bit over 0.1W in this part. But if a tube SHORTS, there could be 1.3 Watts. With a 2-Watt part you change/swap the tube and the gig goes on. With 1W or less, it has to go back to the Tech for smoke repair.
Naturally if you want more work and the client's money is good, you can use 1/4W parts and rattly tubes, be busy every week.
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110uFd sounds like a lot. But if the gig is tonight, and it has been this way a while, leave it.
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Schematic says nothing about anything more than 1/2 watt plate R's but these are "old" carbon comp 1 watts, looks like they've
been there since day one..?
The way I read the schematic, they were all originally 1/2 W 10% tolerance from the factory "if not specified". See note 2. https://el34world.com/charts/Schematics/files/Fender/Fender_super_reverb_ab763_schem.pdf
Am I hallucinating? :icon_biggrin:
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CBS era fenders started using 1W plate resistors. have and had a couple of bassman 100 amps and mid 70's bassman 10 amp - all had 1W plate resistors. 1W can still get noisy and start to crackle with time. i had to replace 3 of the 100K plate Rs on one of those bassman 100.
110uF capacitance for vacuum rectifier anode is too much. it will eventually arc. i would replace them. 2 x 220uF series stack would be adequate for a twin reverb with SSR.
--pete
EDIT: in a pinch you could use one of those SSR tube replacement plug-ins. maybe your local banjo (guitar) center sells them? groove tubes?
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CBS era fenders started using 1W plate resistors. have and had a couple of bassman 100 amps and mid 70's bassman 10 amp - all had 1W plate resistors. 1W can still get noisy and start to crackle with time. i had to replace 3 of the 100K plate Rs on one of those bassman 100.
110uF capacitance for vacuum rectifier anode is too much. it will eventually arc. i would replace them. 2 x 220uF series stack would be adequate for a twin reverb with SSR.
--pete
Ah, so this is a classic Fender "do as I did and not as I said" schematic or this is an earlier schematic I found in the library?
Dude,
Is there a schema on the amp that is different form the one I linked to?
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CBS era fenders started using 1W plate resistors. have and had a couple of bassman 100 amps and mid 70's bassman 10 amp - all had 1W plate resistors. 1W can still get noisy and start to crackle with time. i had to replace 3 of the 100K plate Rs on one of those bassman 100.
110uF capacitance for vacuum rectifier anode is too much. it will eventually arc. i would replace them. 2 x 220uF series stack would be adequate for a twin reverb with SSR.
--pete
Ah, so this is a classic Fender "do as I did and not as I said" schematic or this is an earlier schematic I found in the library?
Dude,
Is there a schema on the amp that is different form the one I linked to?
um, no, '71 fender wouldn't be a AB763 ckt. it's probably the aa1069 or some such. in short, anything past 68 is probably NOT an AB763.
maybe this (https://el34world.com/charts/Schematics/files/Fender/Fender_super_reverb_aa1069_schem.gif) one
--pete
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Well, now I know what I have thanks to Pete, it's an AA1069 made in 1971. 1971 is stamped inside the chassis and the kicker for me is the .002 cap on the output of the reverb, only SR that had that cap. But the amp has been black faced, has the bias setup on 6L6s same as AB763.
Thanks PPR, hear ya on, the 2W plate R's, I found some 100K, 1 watts lying around, I'll use them, the amp had been running years on the old carbon comps, 1 is higher than 1/2, ha.
The schematic calls for a 5U4GB, so does the sticker inside, there was a 5AR4 there. I'll put the 5U4GB back, I assume the PT can handle the voltage draw, it's the original one. Power rail R's were changed too, had a 10K followed by 2K7 after choke, AA1069 and AB763 call for 1K and 4K7, I'll change that back.
The bias for the 6L6's on CBS AA1069 is greek to me, but it's been BF'd, I'll leave it.
I have a single 47uf, 500vdc filter cap, no 70uf's. Last question, can I pull the series 220's @ 350v each and use the one 47UF @ 500? The B+ is 468v. I have nothing for series to get more than 24uf with high voltage rating. Is it better to leave the 220uf @ 700v (110uf) or the 47 @500v? 5U4GB says 40uf
al
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Well, now I know what I have thanks to Pete, it's an AA1069 made in 1971. 1971 is stamped inside the chassis and the kicker for me is the .002 cap on the output of the reverb, only SR that had that cap. But the amp has been black faced, has the bias setup on 6L6s same as AB763.
Thanks PPR, hear ya on, the 2W plate R's, I found some 100K, 1 watts lying around, I'll use them, the amp had been running years on the old carbon comps, 1 is higher than 1/2, ha.
The schematic calls for a 5U4GB, so does the sticker inside, there was a 5AR4 there. I'll put the 5U4GB back, I assume the PT can handle the voltage draw, it's the original one. Power rail R's were changed too, had a 10K followed by 2K7 after choke, AA1069 and AB763 call for 1K and 4K7, I'll change that back.
The bias for the 6L6's on CBS AA1069 is greek to me, but it's been BF'd, I'll leave it.
I have a single 47uf, 500vdc filter cap, no 70uf's. Last question, can I pull the series 220's @ 350v each and use the one 47UF @ 500? The B+ is 468v. I have nothing for series to get more than 24uf with high voltage rating. Is it better to leave the 220uf @ 700v (110uf) or the 47 @500v? 5U4GB says 40uf
al
mostly try to duplicate values used in the schematic, unless the designer(s) were clueless or just plain reckless. fender calls for a stacked 35uF with 510V in standby - can you wait for the correct parts?
i used 2 x 80uF 450V in my SR. i used THESE (https://www.cedist.com/products/capacitor-ft-450v-80-f-axial-lead-electrolytic) parts, with Sprague 20uF 600V parts for the remainder of the power rails.
your 500V part will have 510V on it or MORE even, since mains voltage are typically higher now. if you install a 5AR4, you will have more than 510V on startup and in standby. fender moved away from the 5AR4/GZ34 when CBS took over, and the reasoning for the 750VCT HV secondary on that model - it is to compensate for the higher impedance of the plates of the 5U4GB.
i would not use a 500V part in that amp for the main filter.
if you can't get the correct filter part, then go back to the 5AR4 as it can tolerate a larger capacitive loads and lower source winding impedance.
--pete
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Thanks Pete, I ordered the correct parts. Here's a few guts shots, if you use anything that needs attention let me know.
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your amp schematic specifies a 525V parts for the 20Uf filters - those 500V spragues will have a shorter life than a 525V or 600V parts.
FWIW - i have been using MIEC 20uF 600V, 40uF 600V and 50uF 600V parts lately. seem to be quality parts & have been using them for the last few recap jobs and new builds. the price is an easier pill to swallow than that of the spragues. https://www.studioelectronics.biz/sunshop/index.php?l=product_list&c=100
most customers want the sparagues though... so rather than argue, i let them pay more for a 1/2 hollow cap.
--pete
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Nice prices, thanks. I saw the 525v on the schematic, the Spragues have been running for a while @500v. If 600v e-caps are available now why would anyone use two 350v in series in that doghouse? Just use a 33 or 47uf @600, cheaper? Those are good prices and they have a lot sizes too.
In 1971, who had 600v caps...? Wonder what Fender used in 71 for 525v, I wasn't aware of 525v paper e-caps in 71. Heck, I never saw a 600v cap before you posted. But I hear you on the 525v. rail caps.
al
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Dude the old 525v ecaps I've pulled were Mallory
Pete -thanks for the source and link.. I had been getting those 600v ecaps from Dave in Ontario.. but shipping is slower and pricey.
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I didn't replace the original 525v caps, guy before me did, never worked on an older amp with 525v e-caps never saw that high a rating on the old Mallorys I replaced.
Now I know why the rail r's were higher than the schematic called for, the guy probably only had 500v so he lower the voltage except the first one off the choke, ha. Guess he thought well "only one is in danger of blowing". I would never use a lower rating than a schematic called for, I order 600v caps. But thanks for posting Mallory made old paper 525v caps.
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Nice prices, thanks. I saw the 525v on the schematic, the Spragues have been running for a while @500v. If 600v e-caps are available now why would anyone use two 350v in series in that doghouse? Just use a 33 or 47uf @600, cheaper? Those are good prices and they have a lot sizes too.
In 1971, who had 600v caps...? Wonder what Fender used in 71 for 525v, I wasn't aware of 525v paper e-caps in 71. Heck, I never saw a 600v cap before you posted. But I hear you on the 525v. rail caps.
al
Just about all of the smaller value caps in the Airline I'm working on are 600V rated Sangamos. That was probably 1961? Have them in the 1961 Baldwin organ amp as well. Some larger. Nothing over 1uf though. I have no means of testing (that is non-destructive :l2:) but have heard form those that do that they seldom make it to 600V. Some barely test out at 400V which is why I swapped them out proactively.
When looking for Nichicon's, I ran across the MIEC's from that same seller. Was tempted but knowing nothing about the seller and little about MIEC, decided to stick with the Nichicon and F&T. Maybe next time I will be more adventurous.